1. Field of the Invention
The present invention generally relates to an interferometer using a ring laser; and, particularly, a high accuracy interferometer including external cavity where two beams provided from a ring laser gyroscope through two mirrors are sent back to the ring laser by using external mirrors separated from the ring laser by a certain distance and the length of the external cavity is controlled by a physical quantity to be measured; and, more particularly, an interferometer where the physical quantity is measured by counting the number of pulses generated by interference of two beams propagating in opposite directions in the ring laser, such number being determined by the length of the external cavity. In the interferometer of the present invention, a lock-in zone is broadened by fixing the ring laser and increasing only back scattering of the ring laser so that the interferometer operates in the lock-in zone and the oscillating frequency of the interferometer rarely changes.
2. Description of the Related Art
There are various kinds of laser interferometers which commonly utilize the coherency of laser beams. In a laser interferometer, a laser beam from one source is split into two beams, one of which is made to propagate as a base wave and the other is modulated by a physical quantity to be measured. The two beams are later combined to generate interference fringes whose shape, number, variation in time and interval are assessed to identify the physical quantity such as refractive index, displacement, length and density. Alternatively, known interference fringes are projected to an object so that the shape or surface profile of the object is measured by analyzing the variation of the fringe shape. Typically, in order to get more accurate measurement results, each interference fringe is partitioned into fine sections with equal intervals so that the measurement results may be provided in digital form.
A ring laser gyroscope measures its angular velocity by counting the number of time-variations of beat resulting from a difference of oscillating frequencies of two counter propagating beams in a ring-shaped resonator. Specifically, when a ring laser gyroscope rotates, effective length of the resonator for a beam propagating in the same direction with the rotation is lengthened while that for the other beam propagating in the counter direction is shortened, which phenomenon is known as Sagnac effect. This difference in effective length results in difference of the center frequencies of the two beams so that addition of the two beams provides beat from which angular velocity can be measured in high accuracy.
The accuracy of the angular velocity of the ring laser gyroscope have reached around 0.001 degree/hour. When the ring laser gyroscope rotates below a certain angular velocity, the frequency difference between the two beams is very small so that the two beams oscillate at a same frequency. Such range of low angular velocities is called xe2x80x9clock-in zone,xe2x80x9d which means that the two beams are locked in to each other. The locking-in of the beams results from the fact that mirrors in the ring laser gyroscope scatter part of each beam in the opposite direction from an original direction so that the scattered beam is added to the other beam propagating in the counter direction. This scattered beam is called xe2x80x9cback scattering wave,xe2x80x9d and as the intensity of the back scattering wave becomes larger, the lock-in zone becomes broader. When the ring laser gyroscope is operated in the lock-in zone, it is simply a ring laser because there is neither a frequency difference nor beat. The ring laser is constructed by making a resonator in the form of a polygon having 3, 4 or more sides by using mirrors. There are usually two beams with identical wavelength in the ring laser, one of them propagating clockwise and the other propagating counterclockwise.
An object of the present invention is, therefore, to provide a high accuracy measurement system capable of measuring various physical quantities by using a ring laser and method for using the same.
Another object of the present invention is to provide an interferometer employing a ring laser where the effect of minute movement of the ring laser is minimized on the measurement.
In accordance with one aspect of the present invention, there is provided a ring laser interferometer including an external cavity ring laser where part of each of two beams propagating in opposite directions is outputted from the ring laser through a corresponding mirror and is injected back into the ring laser as back propagating wave by using an external mirror, thereby broadening the lock-in zone. In the ring laser interferometer of the present invention, the length of an optical path, that is, the distance between the external mirror and the ring laser is controlled by the physical quantity to be measured. Then the quantity is identified by counting variation rate in the number of interference fringes which are caused by intensity and phase changes of the two counter propagating waves according to the control of the optical path length.
By inserting an attenuator between the external mirror and the ring laser, the intensity of each back propagating wave can be controlled externally to operate the ring laser only in the lock-in zone. The physical quantities to be measured include displacement, length, position, temperature, refractive index or pressure.
The output from the inventive interferometer is variation rate in interference fringe which depends on intensity sum and difference, and phase difference, of the output beams of the two counter propagating waves, parts of which are fed back to suppress or compensate the effect of fluctuation in applied voltage or other external factors to enable high accuracy measurement. As the ring laser operates in the lock-in zone, the effect of minute movements of the laser on the measurement result can be minimized.
In accordance with one aspect of the present invention, there is provided an optical sensing apparatus for use in a ring laser interferometer for measuring physical quantity, comprising a ring laser cavity resonator; and at least one external mirror, wherein the ring laser cavity resonator includes a plurality of internal mirrors; the ring laser cavity includes two laser beams having a same wavelength and propagating in opposite direction to each other; part of each of the two counter propagating laser beams is transmitted through one of the internal mirrors; and is reflected by one of the external mirrors to generate two back propagating waves, each of which is injected to the ring laser cavity along path of the corresponding transmitted beam through the corresponding one of the selected internal mirrors; and the optical path from each of the selected internal mirrors to the corresponding external mirror is changed based on the physical quantity to be measured.
And also, there is provided an interferometer comprising the optical sensing apparatus described above, at least one platforms for changing the optical paths; a counting unit for counting the number of pulses of a waveform given by variation rate in time of the waveform generated by the intensity difference between the two counter propagating waves in the ring laser cavity based on the change of the optical paths, wherein the physical quantity to be measured is determined from the counted number of the pulses.